1. Field of the Invention
The present invention relates to microcapsules containing a radiation sensitive composition ideal for use in industrial fields which require the detection of ionizing radiation, and to various methods of using these microcapsules.
2. Description of the Prior Art
Attempts to detect radiation by using various compounds which, sensitized by radiation, take on a color or change their color have been made for many years. E.g., compositions made from free radical generators and leuco compounds having a specific structure etc. are known, as one such attempt, from JP, 49-28449, B.
Even though these conventional compositions have the property of taking on a color when sensitized by radiation, there was a problem that, apart from ionizing radiation, they were also sensitized and took on a color when exposed to ultraviolet light, visible light and other types of irradiations.
Consequently, these compositions were not used as a very reliable means for detecting ionizing radiation, since it was not possible to instantly judge, only by confirming the state of coloration, whether or not a coloration was actually caused by ionizing radiation, and since there was a possibility of an accidental exposure to radiation due to a wrong judgment, it was in no way possible to use them for applications having to do with the safety of human beings.
When, in order to solve this problem, these compositions were made so that they are selectively sensitive only to ionizing radiation and non-sensitive (i.e., stabilized) to visible and ultraviolet light, there arose the inevitable technical problem that only extremely high radiation doses are detected and low amounts of radiation remain undetected.
Hence, the present state of affairs is that, despite strong demands from different industries, materials which take on color or which change their colors only when selectively sensitized by ionizing radiation (especially by low doses of radiation) and which are not sensitized by visible and ultraviolet light etc. are still not known.
Thus, having reflected this situation, it is the object of the present invention to provide a means for solving the problems described above by discovering a material which takes on a color or which changes its colors only when selectively sensitized by ionizing radiation (especially by low doses of radiation) and which is not sensitized by visible and ultraviolet light etc.
As a result of an extensive research to solve the above-mentioned problems, the inventors of the present invention have gained the knowledge that, if a specific radiation sensitive composition is microencapsulated, it can be selectively sensitized by ionizing radiation while remaining non-sensitive to ultraviolet and visible light, and have finally completed the invention by conducting further research based on this knowledge.
I.e., the present invention has the following constitutions:
1. Microcapsules containing a radiation sensitive composition comprising leuco compounds (a) and organic halogen compounds (b) as indispensable components.
2. Microcapsules containing a radiation sensitive composition comprising leuco compounds (a) and halogen compounds (b) as indispensable components, and further an organic solvent (c) and/or an antioxidant (d).
3. Microcapsules according to 1 or 2 above wherein the leuco compounds (a) are at least one member selected from the group consisting of triphenylmethane phthalides, fluorans, phenothiazines, phenazines, indolyl phthalides, leuco olamins, spiropyrans, spiro phthalans, spironaphthoxazines, naphthopyrans, rhodamine lactams, rhodamine lactons, indolines, diphenylmethanes, triphenylmethanes, azaphthalides, triazenes, chromeno indoles, xanthenes, diacetylenes, naphtholactams and azomethines.
4. Microcapsules according to 1, 2 or 3 above wherein the leuco compounds (a) are at least one member selected from the group consisting of Compounds 3 and 4 below; 
wherein each of the rings A, B, C, D, E represents 5 or 6 membered aromatic or heterocyclic rings which may have a substituent group, one or more aromatic or heterocyclic rings may be condensed, and each of the rings may be cross-linked by O, N or S. Further, Y exists when the central carbon is not conjugated with the D or the E ring, and represents hydrogen, a hydroxy group, an alkoxy group or an aryloxy group.
5. Microcapsules according to 1, 2, 3 or 4 above wherein the organic solvent (c) is at least one member selected from the group consisting of alcohols, esters and aromatic hydrocarbons.
6. Microcapsules according to 1, 2, 3, 4 or 5 above wherein the microcapsules take on a color or change their colors when sensitized by dose levels of radiation in the range from 0.001 to 20000 Gy.
7. Radiation sensitive ink or paint containing the microcapsules according to any of 1 to 6 above.
8. A radiation detecting method for detecting whether a substrate onto which the ink or paint according to 7 above has been printed or painted is exposed to radiation or not.
9. A radiation sensitive indicator made from a carrier containing at least the microcapsules according to any one of 1 to 6 above.
10. A radiation sensitive indicator wherein a radiation sensitive layer containing at least the microcapsules according to any one of 1 to 6 above is formed on a carrier.
11. A radiation sensitive indicator wherein a radiation sensitive layer containing at least the microcapsules according to any one of 1 to 6 above, and an ultraviolet light absorption layer and/or a visible light absorption layer is formed on a carrier.
12. A radiation detecting method for detecting radiation on a substrate onto which the radiation sensitive indicator according to 9, 10 or 11 above has been attached.
The present invention was made based on the remarkable new discovery that a radiation sensitive composition made from leuco compounds (a) and organic halogen compounds (b), even though in itself photosensitive and sensitive to any type of irradiation, can be made to be selectively sensitive to ionizing radiation and non-sensitive to ultraviolet light, visible light and other types of irradiations.
The most significant feature of the present invention is that a radiation sensitive composition of a specific composition is enclosed in microcapsules, and that as a result of this microencapsulation said composition becomes selectively sensitive to ionizing radiation and non-sensitive (stabilized) to visible and ultraviolet light etc.
It is presumed that the reason why the present invention exhibits such a selective effect is probably because by enclosing said composition in microcapsules, which are mininute discrete and closed systems, it is stabilized by the effect of these particular systems; however, the precise mechanism thereof is still not understood.
Anyway, this selective effect of the present invention is a novel and special effect, unknown until the present day, which occurs as a result of the microencapsulation.
The present invention, by means of this special feature, provides a material which takes on a color or changes its colors, especially, when selectively sensitized by low radiation doses in the range from 0.001 to 20000 Gy, which is impossible by conventional means. Such a material has tremendous industrial utility.